An optical amplifier is for optically amplifying a signal directly to compensate for a loss occurring when an optical signal propagates in an optical fiber transmission path in an optical transmission system and further for losses in optical parts/optical modules, and it has an optical fiber medium for the optical amplification and an equipment for supplying pump light. This pump light supplying equipment supplies pump light with a predetermined wavelength to the optical fiber medium for the optical amplification, thereby placing the optical fiber medium into an activated state. The signal light is optically amplified and outputted when a signal is inputted to this optical fiber medium placed into the activated state.
In a recent-year optical amplification repeating transmission system, a rare earth doped optical fiber amplifier in which a rare earth element ion such as Erbium is doped and a Raman amplifier utilizing a Raman induced scattering characteristic of an optical fiber have been applied as an optical fiber amplifier medium. In particular, a long-distance large-capacity transmission system based on a wavelength multiplexing (Wavelength Division Multiplexing; WDM) transmission technique, attention has been paid to a Raman amplification technique for the purpose of the improvement of performance, and an optical repeater has frequently been configured on the basis of a combination thereof with a rare earth doped optical fiber amplifier represented by an EDFA (Erbium Doped Fiber Amplifier).
Although an optical fiber serving as a transmission path and a dispersion compensation fiber which compensates for dispersion in an optical amplification repeating transmission system has merely being a loss medium so far, owing to the realization of an pump light source capable of outputting a single mode of several hundreds mW, the introduction of a Raman amplifier expectable to improve the OSNR characteristic has been in progress. That is, since the amplification medium of this Raman amplifier has employed a transmission path optical fiber or dispersion compensation fiber which has been a loss medium so far, the effective improvement of the OSNR in a transmission path is expectable.
In a case in which a quartz-based optical fiber is used as an amplification medium, the Raman amplification has a peak at a lower frequency by approximately 13.2 THz (approximately 100 nm when the excitation wavelength is a 1.4 μm band) than an pump light frequency and, in most cases, it has an asymmetrical optical amplification band. Moreover, it is expectable that optical amplification bands are superposed by introducing pump lights with a plurality of different wavelengths into an optical fiber so as to secure the flattening of the output light level according to wavelength for achieving a broadband signal light amplification.
As optical amplification repeaters in the actual optical amplifier repeating transmission system, there have been reported a configuration of an amplification repeater based on a combination between an EDFA and a Distributed Raman Amplifier (DRA) in which a transmission line serves as an amplification medium and excitation wavelengths, which are 2 or 3 in number, are introduced thereinto to realize the same bandwidth as that of an EDFA with a wavelength of 30 nm, a configuration of a dispersion compensation fiber Raman amplifier (DCFRA) designed to accomplish the Raman amplification on a dispersion compensation fiber, and other configurations (see Patent Document 1).
Thus, the mainstream of the Raman amplifier has been a configuration in which a large number of excitation wavelengths are available for the purpose of broadbandization of a wavelength band functioning as an amplification band. Accordingly, the optical amplification repeater has a Raman pump light source in addition to the pump light source for the EDFA, so the power dissipation for the pump light sources tends to increase and the packaging area tends to increase due to the heat sink for the thermal emission. For this reason, the optical amplifier requires the reduction of power dissipation of the excitation sources.
Furthermore, different transmission distances and transmission line losses occur in a land optical transmission system, and the signal level to be inputted to an optical amplification repeater stands at a different value for each transmission span. For the design of an optical amplifier, when an optical amplifier is designed so as to maintain the flatness characteristic of an output wavelength according to such a signal input level range, a large amount of optical amplifier menu takes place, which creates undesirable problems in management cost, increase in stock and other problems. For this reason, in most cases, an optical amplifier requires a wide signal input power dynamic range while maintaining the flatness characteristic.
In particular, in the case of a Raman amplifier, since a tilt characteristic largely varies depending upon a wavelength arrangement of an inputted optical signal due to a Raman effect between optical signals, for carrying out the Raman amplification employing pump light with a plurality of wavelengths, there is a need to supply pump light with a plurality of wavelengths in a power ratio so as to maintain the flatness characteristic of an output signal light wavelength according to a wavelength arrangement of an inputted optical signal. In a case in which a rare earth doped fiber amplifier is located at the latter state of a Raman amplification medium, there is a need to consider a tilt characteristic of an output of this rare earth doped fiber amplifier.
Among conventional optical amplifiers, there is an optical amplifier designed to control a Raman gain by controlling a branch ratio of one-system pump light and an excitation intensity while supplying this pump light to a fiber amplifier and a Raman amplifier for the purpose of securing the aforesaid signal input power dynamic range (see Patent Document 2).
As well-known other techniques related to the invention of the subject application, there are techniques disclosed in the Patent Documents 3 and 4.
Patent Document 1: Japanese Patent Laid-Open No. 2000-98433
Patent Document 2: Japanese Patent Laid-Open No. 2003-283019
Patent Document 3: PCT Japan National Publication No. 2004-511004
Patent Document 4: Japanese Patent Laid-Open No. HEI 11-84440